America doesn't need new cities. There is an unbelievable amount of space wasted within every city and any urban planner can show you. Development policy in the 40s and 50s was backward and we continue to suffer the consequences today.
It can be both. A well developed new city will be faster and easier to build. Retrofitting an existing city bumps up against existing infrastructure (we have to move these pipes and electricity lines) and the individual property owners. My grand father owns the last remaining house with a few acres in a dense office park. He is single handedly blocking about a billion dollars of new development.
Absolutely, but it's easier to build quality mixed use development and walkable, bikable neighbourhoods when you don't have thousands of NIMBYs screaming down every proposal.